How to do? With a fine brush, I apply the pigments without crushing them too much on the surface. The point is to have enough pigments to get a gritty look, like dust that would have gone through some rain for example. And that's all there is: make some fixer rain on the pigments to give them an irregular look!

I always start with light colors to then darken the effects. Then I fill my airbrush with Tamiya thinner. I've tried using AK-Interactive Pigment Fixer, which works very well, but I think it's a bit sticky and gives (in my opinion) a thicker effect. It works fine on 1:35 but not on 1:48. Then I put a low pressure in the compressor and I place the airbrush to a fair distance from the model and simply make the thinner rain on the pigments. Once they start to adhere to the surface, I can start getting closer to the model with the airbrush without drowning the pigments! Once dry, the gritty aspect appears!

You can also use the pigments in a more traditional way on the vertical surfaces like under the mudgards. You apply the pigments and then add some thinner by capillarity! The effects I have are correct, but I need to work on them again!